When casing is being run into a wellbore and the annulus outside the casing is subsequently being cemented, the casing is connected to a cementing head for cement to be fed into the annulus. When subsea structures are being cased, a landing-pipe string extends between the upper end portion of the casing and the installation from which the drilling takes place, typically a platform projecting up above the sea surface. With the technical solutions of today, when the operations are taking place from a floating vessel, the casing must be hung off with a safe distance to the planned setting depth, the distance depending on the expected heave motions of the drilling vessel. The normal distance is typically 15-25 meters. The cementing head must then be connected to the landing-pipe string at an even greater height above the rotary table on the drilling floor, and this is a time-consuming and hazardous operation, which takes place partially with personnel hanging in belts from the derrick. The cementing head will often be hanging from the derrick for some considerable time, waiting to be connected, and a situation with hanging objects above the drilling floor is not desirable from a safety point of view.
When the casing remains hanging above the setting depth for a long time, there is always a risk that there may be a blocking of the casing against the borehole wall, so that further movement to the planned setting depth is impeded.
The drawbacks of this technique is partially remedied by using a tubular, telescopic pipe-landing unit as it is described in the applicant's own NO-patent 328917. It has turned out that the use of this pipe-landing unit has resulted in a substantial simplification of the operations connected to the landing of borehole installations from a floating surface installation, but great challenges still attach to the maintenance of such pipe-landing units when they are used in connection with cementing operations. During the pumping of cement through the landing string, the annulus between an inner pipe-landing section and an outer pipe-landing section will become filled at least partially with cement, and after the operation has been carried out, the prior-art telescopic pipe-landing unit must therefore be thoroughly cleaned, an operation, which is time-consuming and costly. With poor cleaning, cement residues will set in the annulus, which may ruin the telescoping function of the unit or the seal systems of the unit.